1. Field of the Invention
This invention relates generally to clear air turbulence detection means and more particularly to an improved system and method for detecting regions of clear air turbulence through detection of infrared radiation in the water vapor rotational wavelength band.
2. Description of the Prior Art
Clear air turbulence, often termed CAT, is a region of high turbulence encountered by an aircraft without visual or radar warning. CAT includes all forms of turbulence occurring in clear air which do not involve convective forces.
Two separate conditions are known which result in a CAT. The first condition is created by a standing wave found in the lee of a mountain barrier which occurs when statically stable air is carried over the mountains. The second condition results from waves formed in statically stable layers in the atmosphere that are subjected to sufficiently strong vertical wind gradients (shear). These shear-induced waves are commonly referred to as Kelvin-Helmholtz (KH) waves.
It is virtually impossible to predict in advance when and where an individual CAT will occur. P. C. Patnaik, F. S. Sherman and G. M. Corcos, "A Numerical Simulation of Kelvin-Helmholtz Waves of Finite Amplitude," Journal of Fluid Mechanics; 73(2n), 215 (1976). There are, however, several approaches for CAT detection. Prior art approaches for detecting CAT have been based primarily on temperature gradients existing in a CAT. Temperature gradients have been priorly detected by receiving microwave or infrared energy from the CO.sub.2 band of the spectrum with a radiometric receiver and processing that information. One such temperature gradient approach was set forth by R. W. Astheimer in Applied Optics, Vol. 9, page 1789 (1970).
U.S. Pat. No. 3,696,670 issued to Collis on Oct. 10, 1972 entitled "Method and System for Detecting Clear Air Turbulence" summarizes these prior art approaches as follows:
"In order to provide remote warning of clear air turbulence that is directly associated with horizontal temperature gradients a number of radiometric techniques have been proposed and tried to detect such gradients ahead of an aircraft. One prior art system accomplishes this by radiometric scanning in frequency on the skirts of the CO.sub.2 absorption band in order to provide an instantaneous reading of temperature variation along the aircraft path, which will thus show anomalies in gradients. In another prior system there is provided at each instant a single reading of effective temperature which is integrated over the whole path ahead of the aircraft and time history is relied upon to remotely reveal anomalies in the temperature gradients in the direction of the aircraft's path. These systems have, however, had disadvantages. They have been complex and expensive and have required a reference, such as black body for example, with which to compare the signals received by the radiometric receiver or radiometer. There have also been problems in stabilizing the airborne radiometers. Inasmuch as temperature varies considerably with vertical distance or altitude small pitch variations in the radiometer's alignment produces substantial error and noise into its readings. (Column 1, lines 41-64)."
Collis, thereupon, set forth a method and system which utilized at least two remote atmospheric regions ahead of the aircraft's path for detecting CAT. The temperatures in these remote atmospheric regions were detected and compared according to a disclosed processing arrangement. Collis essentially required duplication of equipment by requiring the presence of at least two radiometers. The approach set forth by Collis relied upon the temperature difference sensed in at least two atmospheric volumes.
All of the above prior art approaches used a horizontal or vertical temperature gradient detection process. However, it has been recently postulated that temperature gradients obtained from aircraft temperature sensors do not correlate with CAT statistics. M. A. Bender, H. A. Panofsky and C. A. Pesten, "Temperature Gradients and Clear Air Turbulence," Journal of Applied Meteorology 15, 1193-1199 (1976).
The main problem with these prior art approaches relates to the unsatisfactory high failure rate of detection. Collis in his U.S. Pat. No. 3,696,670 suggested the use of a detector based on water vapor anomalies (Column 5, lines 39-43). Subsequently and without knowledge of Collis' suggestion, the inventor accidentally discovered the detector of CAT through detection of anomalies in the water vapor band. This event was published in Elson, "Radiometer Studied as Turbulence Sensor", Aviation Week (May 12, 1975). Prior to this discovery the inventor with others was performing research into atmospheric water vapor:
(1) P. M. Kuhn, M. S. Lojko and E. V. Petersen, "Water Vapor: Stratospheric Injection by Thunderstorms", Science, Vol. 174, pages 1319-1321 (Dec. 24, 1971).
(2) P. M. Kuhn and L. P. Stearns, "Radiometric Observations of Atmospheric Water Vapor Injection by Thunderstorms", Journal of Atmospheric Sciences, Vol. 30, pages 507-509 (April, 1973).
(3) P. M. Kuhn, "Zonal Profiles of Atmospheric Water Vapor", NOAA Technical Report, ERL 319-APCL 33 (March 1975).
(4) P. M. Kuhn, L. P. Stearnes, M. S. Lojko, "Latitudinal Profiles of Stratospheric Water Vapor", Geophysical Research Letters, Vol. 2, page 227 (June 1975).
(5) P. M. Kuhn, E. Magaziner, and L. P. Stearns, "Stratospheric Areal Distribution of Water Vapor Burden and the Jet Stream", Geophysical Research Letters, page 529 (September 1976).
In "Clear Air Turbulence: Detection by Infrared Observations of Water Vapor", Science, Vol. 196, 1099-1100 (June 3, 1977), the inventor set forth an operational CAT-sensing radiometer detecting system detecting the infrared radiation from the water vapor rotational bands at 6.3 micrometers and at 19.0 to 37.0 micrometers. The radiometer system had as its goals (1) to develop a simple water vapor and radiometer system of modest cost that can operate unattended and can achieve accuracy in alerting crews to CAT encounters from 4.0 to 10.0 minutes before the event and (2) to study the most probable mechanisms which allow the passive detection of CAT in the water vapor IR bands. This CAT radiometer tentatively considered use of the water vapor IR bands in the 19.0 to 37.0 micrometer range with a forward looking and directed upward sensor to 2.5.degree. to 7.5.degree. fixed elevation angle from the airplane path. Initial results indicated an 80% CAT detection with a 4-7 minute warning of the impending CAT. However, the results obtained were general, and further experimentation on the band width, field of view, and elevation angle was deemed, in the article, to be warranted. Furthermore, the severity of CAT was suggested as being possible of measurement although no system or method was proposed. The detection system set forth in the above article, however, was a proof-of-the-principle apparatus and has, now, been significantly improved upon under the teachings of the present invention.